This invention relates to endless positive drive transmission belts. In one aspect, the invention relates to an improved belt tooth construction. In another aspect, the invention relates to an improved method for making toothed belts. In a further aspect the invention relates to an apparatus for making an improved toothed belt.
Power transmission belts used with toothed pulleys are well known in the art. These belts have a plurality of alternating teeth and grooves extending generally transversely of the belt which mesh with alternating teeth and grooves of the toothed pulley or sprocket in order to perform their driving function. The most widely used of these toothed belts are so-called synchronous or positive drive belts which are manufactured from flexible resilient material such as natural or synthetic rubber. These belts are engineered and manufactured with pitch, tooth depth, width and other measurements accurate to a precise degree of extremely close tolerances being maintained. In addition, a high strength tensile stress-resisting member of essentially inextensible material is provided substantially on the dedendum line of the teeth to prevent undue stretching of the belt. This belt construction allows the flexible, resilient belt teeth to mesh without substantial change of pitch with teeth of the toothed pulleys, with the belt thereby functioning as a synchronizing belt. The operation and advantages of synchronous drive belts are fully described in Case, U.S. Pat. No. 2,507,852.
Conventionally, in the prior art, these synchronous belts have been made by one of the following methods: (a) the extruded tooth method; (b) the tooth preform method; and (c) the flow through method.
The extruded tooth method, as described by Case, U.S. Pat. No. 2,507,852 comprises the steps of lining a grooved mold with fabric, filling the grooved spaces with strips of an unvulcanized, plasticized rubber compound which forms the body of the belt teeth, helically winding a load-carrying member around the outer ends of the mold lands, applying a layer of rubber compound over the load-carrying member and curing the assembly under pressure.
The advantage of this system is that the elastomeric material used for the toothed portion of the belt can be made of high modulus material to resist deformation under load, while the back of the belt can be made of material designed for optimum flex-fatigue resistance. Belts made by this technique are generally of low quality. Contamination (and semi-cure) due in large part to the extrusion process used to form the tooth material results in low levels of adhesion between the fabric facing and the tooth material and more importantly between the tooth material and the load-carrying member of the belt. Further, very little movement of rubber occurs during vulcanization and release agents applied to the extruded tooth material prevent the formation of good chemical bonds between elements even when the greatest care is taken to prevent contamination. Additionally, if the volume of the extruded tooth material is not carefully controlled the load-carrying cords will be wound on the tops of the extruded tooth material instead of the mold land area. This increases the pitch length of the belts produced and makes accurate length control difficult to achieve.
The tooth preform method as disclosed by Geist et al., U.S. Pat. No. 3,250,653 and Lindner et al, U.S. Pat. No. 3,973,894, comprises the step of forming an enveloping fabric in precise conformity with the shape of the teeth and filling in the formed hollow folds with a plastic raw mixture. This preform is placed upon the tooth core of a suitable belt building device and the remaining components are built up on the building device.
The advantage of this method is that contamination is reduced considerably because of the wiping action of the rubber during preforming. Contamination at the interface between the load-carrying member and the preform still limits the performance of these belts and length control is an even greater problem than with the previous method. Very frequently a layer of rubber remains in the land area of the preform and this results in the load-carrying member being wound on a larger circumference than the designer intended. Once again length control becomes very difficult.
Belts made from either of the above methods have stock interfaces directly beneath and adjacent to the load-carrying member of the belt. This is the area of highest shear stress in the belt and at the same time it is also the area likely to have the poorest bond strength.
The flow-through methods, as described by Skura, U.S. Pat. No. 3,078,206 comprises the steps of wrapping a jacket fabric around a grooved mold, winding a tensile member over the jacket, wrapping a layer of rubber over the tensile member, and then forcing a portion of the backing layer through the tensile member into the grooves of the mold to form the belt teeth. In this operation, the jacket fabric is stretched by the mold rubber and conforms to the contour of the grooved mold.
This method of manufacture overcomes the adhesive interface problems associated with the previous two methods because the belt backing material extends through the tensile member layer into the belt teeth. A further advantage of this system is that the grain of the rubber in the teeth is oriented to resist shear failure. The greatest problem with this method is that the elastomeric material used in this belt must be designed for the needs of the manufacturing process rather than for the finished product. This puts severe limits on the performance capability of these belts. High strength high modulus materials cannot be successfully molded through the neutral axis layer, thereby limiting the strength and modulus of the belt teeth.
The belt teeth of synchronous belts are subjected to a very high shear stress and it has been found that the greatest belt failure occurs as a result of the complete shearing of the teeth from the remaining belt structure. Many different elastomeric materials, as well as different tooth and pulley configurations, have been proposed or utilized to reduce or eliminate the amount of shear to which the belt teeth are subjected.
It has been proposed to alter the tooth construction by incorporating reinforcing means into the matrix of the relatively weaker elastomeric material. The elastomeric matrix in which the fibers are embedded serves to transmit the load from fiber to fiber by shear. Since the length/diameter ratio of the fiber reinforcing material is large, the shear load of the matrix is low and consequently, the fiber becomes the highly loaded component of the composite structure.
When making toothed belts by the extruded tooth method or the tooth preform method, incorporation of fiber reinforcing material into the elastomeric matrix presents no real problem. The flow-through method, however, requires elastomeric compounds having relatively low compound viscosities. Incorporation of fiber reinforcing material into the elastomeric matrix increases the compound viscosity. Miller, U.S. Pat. No. 3,535,946, discloses that toothed belts having fiber reinforcement in at least a portion of the tooth can be made using the flow-through method. In practice, however, it has been found that the tensile member must be spaced apart enough to allow the fiber-filled elastomeric matrix to flow through, thereby reducing the tensile strength of the belt.
It is therefore, an object of the present invention to provide an improved toothed belt.
Another object is to provide an improved method for making toothed belts.
A further object is to provide an apparatus for making toothed belts.
These and other objects, aspects and advantages of the present invention will appear more clearly from the following specification in connection with the accompanying drawings.